Penning type gauge head with ignition aid

ABSTRACT

A penning type gauge head with an electrode system comprising an anode and a cathode disposed in the magnetic field of a permanent magnet, including an ignition aid for the electrode system. In order to provide rapid and reliable ignition, the ignition aid consists of a least one metal strip attached on the anode, and the free end of the ignition aid extends in the direction toward the cathode.

BACKGROUND OF THE INVENTION

The invention relates to a Penning type gauge head with an electrodesystem comprising an anode and a cathode disposed in the magnetic fieldof a permanent magnet, as well as with an ignition aid for the electrodesystem.

Penning type gauge heads of this type are installed in ionization vacuumgauges. They comprise two unheated electrodes (cathode and anode)between which, by means of a DC voltage of the order of magnitude of 2kV, a cold discharge is ignited and maintained. With the aid of amagnetic field the path of the electrons is extended so that theirimpact rates with gas molecules is sufficiently large in order to formthe required number of charge carriers for maintaining the dischargeeven at very low pressures. The positive and the negative chargecarriers migrate to the corresponding electrodes and form thepressure-dependent discharge current which is displayed on a measuringscale. Of disadvantage in vacuum gauges of this type is that thedischarge cannot always be ignited.

From DE-OS 30 47 744 a gauge head of the above described type is known.To attain a reliable ignition it is suggested to provide, apart from themeasuring system comprising unheated electrodes, a glow cathode inaddition. This measure is expensive since the supply of a sufficientlylarge current to the glow cathode is required. An additional currentsupply device as well as a separate current lead-in are required.

From EU-A-271 769 a gauge head of the type relevant here is known,which, apart from the measuring system (main measuring system),comprises a further Penning system with cathode and anode (ignitionsystem). Its dimensions are significantly smaller than the dimensions ofthe main system. The production of an ignition system of this type isalso expensive. Moreover, tests have found that the ignition of themeasuring system is not always ensured. The gauge head known fromEU-A-271 769 was therefore never introduced on the market.

SUMMARY OF THE INVENTION

The present invention is based on the objective of equipping a gaugehead of the above cited type with a reliable ignition aid of simplestructure.

According to the invention, this objective is accomplished by attachingat least one metal strip on the anode, whose free end extends in thedirection toward the cathode. Due to the high electric field strength inthe region of the free end of the metal strip, gas ions are detachedwhich impinge on the cathode and generate their secondary electrons.These cause a rapid and reliable ignition of the electrode system.

A preferred embodiment of the present invention has been found by theselection of the longitudinal direction of the metal strip such that itis not parallel to the magnetic lines but rather intersects the magneticlines. The ignition could be further increased significantly throughthis measure.

Further advantages and details of the invention will be explained infurther detail in conjunction with an embodiment example depicted inFIGS. 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

The Penning type gauge head 1 shown schematically in FIGS. 1 and 2comprises a vacuum-tight housing 2 which simultaneously forms thecathode.

The housing 2 is equipped with a flange 3 with which the gauge head isconnected to a receptacle in which the pressure is to be measured and/ormonitored.

Within the volume 4 formed by the housing, respectively the cathode, 2is disposed the annular anode 5 whose axis is denoted by 6. With the aidof the anode voltage feed 7 the anode 5 is mounted in the electrodevolume 4. The anode voltage feed is at a positive high-voltage potential(approximately 2 kV). During the operation of the gauge head thedischarge is maintained between the anode 5 and the grounded housing 2.

Anode 5 and cathode, respectively housing, 2 are disposed in a magneticfield generated by magnets 8 and 9. One north pole and one south poleeach are facing the housing 2 so that the magnetic lines of the magneticfield extend approximately parallel to the anode axis 6.

The ignition aid consists of at least one metal strip 11 which isattached centrally in the region of the anode voltage feed 7 on anodering 5. The free ends of the ignition aid project laterally from theopenings of the anode ring 5. The free ends extends up to the proximityof the cathode, respectively housing, 2. The high electric fieldstrength at this site detaches gas ions from the ends of the metalsheet. By impinging these ions onto the housing wall secondary electronsare generated. Through these electrons the fast and reliable ignition ofthe Penning type gauge head is possible even at low pressures.

The sheet metal strip usefully comprises a 0.1 mm thick weakly magneticmaterial. The distance from the housing wall is 0.5 to 1.0 mm at adiameter of the ring anode 5 of approximately 50 mm. It is understoodthat it is also possible that the ignition aid comprises two separatesheet metal strips which are each attached with their one end at theanode ring 5. But one metal strip of this type alone is sufficient asignition aid.

In order to achieve that the metal strips 11 forming the ignition aid donot extend parallel to the magnetic lines which conventionally areparallel to the anode ring axis 6, the free ends of the sheet metalstrip(s) 11 are bent for example in the direction toward the ring axis 6after they are attached on the anode ring 5. An angle of approximately20° between metal strip 11 and ring axis 6 was found to be useful.

I claim:
 1. A penning type gauge head with an electrode systemcomprising an anode and a cathode disposed in the magnetic field of apermanent magnet, further including an ignition aid for the electrodesystem, wherein said ignition aid comprises at least one metal stripattached on the anode with the free end of said strip extending in thedirection toward the cathode.
 2. A gauge head according to claim 1wherein the metal strips comprise a weakly magnetic material.
 3. A gaugehead according to claim 1 wherein a ring forms the anode, a housingwhich encompasses the ring forms the cathode, and whereby the metalstrip is attached in the region of the anode voltage feed on the anodering.
 4. A gauge head according to claim 3 wherein two metal stripsproject laterally from the openings of the anode ring.
 5. A gauge headaccording to claim 3 wherein a metal strip is attached centrally on theanode ring, and that its ends project from the openings of the anodering.
 6. A gauge head according to claim 5 wherein said ends extend tothe proximity of said cathode.
 7. A gauge head according to claim 3wherein the longitudinal directions of the metal strips are selectedsuch that the metal strips intersect the magnetic field lines.
 8. Agauge head according to claim 7 wherein the angle between thelongitudinal direction of the metal strips and the magnetic field linesis about 20°.
 9. A gauge head according to claim 8 where said metalstrips are bent toward said ring.